1. Field of the Invention
The present invention relates to a method and an apparatus for generating an origin signal of an encoder that has a scale on which an incremental pattern and an origin pattern are formed. In particular, the invention relates to a method and an apparatus for generating an origin signal of an encoder, by which the origin signal can be generated with high precision irrespective of origin waveforms obtained from the origin pattern.
2. Description of the Related Art
FIG. 1 shows an encoder that obtains an incremental displacement signal (referred to as main signal) by using a scale 10 on which an incremental pattern 12 is formed. An origin pattern 14 is formed beside the incremental pattern 12 if it is necessary to acquire the point of origin on the scale 10. As shown in FIG. 2, an origin waveform is obtained from this origin pattern 14, and a pulsed origin waveform signal is then determined (see Japanese Patent Laid-Open Publications Nos. 2000-97726, 2000-275063, 2003-294494, and 2004-16330).
In FIG. 1, the reference numeral 20 designates a detection head which includes a light source 22, an index scale 30, four main signal light-receiving devices 42, and an origin light-receiving device 44. The index scale 30 is provided with patterns 32 and 34 corresponding to the incremental pattern 12 and the origin pattern 14, respectively. The main signal light-receiving devices 42 acquire respective four phases of output for the sake of direction discrimination and phase division.
This method is highly tolerant to mounting and relative-movement plays such as in the gap, arrangement, and orientation of the detection head 20 with respect to the scale 10. Nevertheless, the method also has had a problem of poor performances in use, including repeatability and bidirectionality.
In the meantime, encoders have improved in resolution and precision recently, and a demand for origin signals of higher precisions has become increasingly larger. It has thus been desired that the origin signals have resolution and precision equivalent to those of main signals, and be output in synchronization with the timing of the main signals as well. Japanese Patent Laid-Open Publication No. 2003-83771 describes the acquisition of an origin signal synchronous with a main signal.
Even with the technique described in Japanese Patent Laid-Open Publication No. 2003-83771, however, it has been impossible to output an origin pulse that is adaptable to necessary origin pulse widths.